1. Field of the Invention
The present invention relates to the structure of a starter which starts up an engine.
2. Description of the Related Art
In order to reliably perform opening and closing of a movable contact and a fixed contact of an electro agnetic switch, there is heretofore know a starter in which a spring is disposed between a lever that pushes out a pinion and a plunger. In such a starter, a configuration is made such that the plunger is sectioned by actuation of the electromagnetic switch and the lever engaged with the plunger pushes out the pinion; and thus, the pinion is meshed with a ring gear (for example, Patent Document 1).
In the aforementioned starter, the suction force of the plunger is strong against inertia of the pinion, an overrunning clutch to be pushed out in an axial direction together with the pinion, and the lever; and therefore, the spring may begin to be compressed in the initial motion of the plunger, that is, at the time when the plunger begins to be suctioned.
If the spring begins to be compressed in the initial motion of the plunger, the contacts close before the pinion is meshed with the ring gear; as a result, a problem exists in that the motor starts rotation, the motor rotation is transmitted, the rotating pinion and the ring gear are not meshed well, and what is called a meshing defect is generated.
As this countermeasure, generally, there includes a method which strengthens the spring; however, if the spring is strengthened, a drawback exists in that electromagnetic force of the electromagnetic switch, which is for overcoming against the countermeasure, needs to be increased and the electromagnetic switch increases in size. In addition, a problem exists in that if the electromagnetic force is increased, the momentum of the plunger is also increased and accordingly the spring needs to be further strengthened; and an effect by the method of increasing the electromagnetic force is slightly obtained regardless of the increase in size of the electromagnetic switch.
Furthermore, as another countermeasure, there is known one in which a starter includes: a first shaft disposed inside a plunger; a drive spring incorporated between the plunger and the first shaft; and a second shaft mounted with a movable contact and disposed inside the plunger. In the starter, the second shaft is rotatably borne to an inner diameter section of the first shaft so as to be a negative pressure by generating a space section between both of the shafts when both of the shafts move in an axial direction apart from each other at the time of actuation of the starter (for example, Patent Document 2). In this starter, airtightness of the space section between both of the shafts is enhanced so that an air damper function operates at the time of actuation of the starter and thus the momentum of the movement of the second shaft is suppressed and the momentum of the plunger is suppressed; and therefore, the contacts do not close before the pinion is meshed with the ring gear and consequently an effect can be obtained in that meshing property is improved.    [Patent Document 1] Japanese Unexamined Utility Model Publication No. Hei 2-57535 (FIG. 1)    [Patent Document 2] Japanese Examined Patent Publication No Hei 3-47430 (FIG. 2)
However, in the structure of the electromagnetic switch of the starter disclosed in Patent Document 2, the space section between both of the shafts enhances the airtightness by processing; and therefore, the space section is subject to variation in the negative pressure, an effect that suppresses the momentum of the plunger is unstable, and there cannot be stably obtained an effect that meshing property of the pinion and the ring gear is improved.
Furthermore, because of the structure which indirectly suppresses the compression of the drive spring by suppressing suction (movement) speed of the plunger, in order to sufficiently obtain the aforementioned air damper function, extremely high airtightness is required for the space section between both of the shafts, so that high process accuracy is needed and thus it could be factors of increased costs.